drm/xe: Annotate each dumpable vma as such

In preparation for snapshot dumping, mark each dumpable VMA as such, so
we can walk over the VM later and dump it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240221133024.898315-4-maarten.lankhorst@linux.intel.com